The invention relates to a process for producing diagnostic quality x-ray images from a fluoroscopic sequence. more particularly, the invention relates to a system by which the images of a fluoroscopic sequence are combined to form an image having superior quality when compared to the individual images from the sequence.
Trraditionally, x-ray imaging has been divided into two major classifications: radiography and fluoroscopy.
Radiography generally involves the transmission of x-rays through a subject, wherein x-ray photons that are not absorbed by the subject reach a receptor and form a "shadow" of the subject. The resulting static image is typically acquired in less than one second on the receptor, which may be a film-screen combination, or a solid state detector. If properly acquired, a relatively sharp, high resolution image results which is approved for a variety of diagnostic purposes.
Fluoroscopy also involves the transmission of x-rays through a subject. However, x-ray photons that are not absorbed by the subject strike a fluorescent screen, wherein a continuous image is recorded therefrom with a television camera or digital camera. The resulting real-time sequence of images can display movement. The images are typically acquired and displayed at up to thirty frames per second. Typically the image quality of a fluoroscopic image, or a sequence of fluoroscopic images, is limited by the relatively smaller number of photons reaching the receptor during the relatively short exposure time therefor.
Mammograms are conventionally not performed using fluoroscopy since fluoroscopic images inherently lack the quality necessary for critical examination. Thus, radiography is used which requires a long exposure time. To prevent motion artifacts from motion during the exposure, the breast is compressed, resulting in severe discomfort to the woman. In addition, because of the length of exposure, effective mammography in mobile screening centers is greatly reduced--making mammograms less accessible to the poor, and woman in rural or remote locations.
Generally, radiographic quality is compromised by a number of "artifacts". Artifacts are undesirable features of an image which make using the image for diagnostic purposes more difficult. Among these are motion artifacts and quantum mottle. Motion artifacts result from movement of the patient, especially during pediatric radiography, and in theory can even result from vibration of the image acquisition equipment. Quantum mottle occurs when insufficient photons reach the receptor, and quantization occurs to flatten the image or create a mere shadow of the subject--devoid of detail. Quantum mottle is especially pervasive with fluorography, where short exposure times can rob images of crucial detail.
Other factors which influence the usefulness of an x-ray image for a particular diagnostic purpose include the interaction between different types of human tissue with the x-ray photons. In particular, x-ray photons generated with a different "kV setting" will penetrate different tissue in varying degrees. Accordingly, different kV settings will reveal different features and abnormalities in different types of tissue. However, because conventional x-rays are acquired using a single kV setting, each image is in effect a compromise of which anatomical features to emphasize.
With regard once again to fluoroscopy, several attempts have been made to increase the usefulness of fluoroscopic images. However, by there nature, these techniques are not suited for the creation of images of diagnostic quality.
One such technique, known as "image hold" attempts to improve the quality of final fluoroscopic image through the averaging of several individual frames. However, because there is no correction for motion, increasing the number of individual frames will blur an image having motion artifact.
While these units may be suitable for the particular purpose employed, or for general use, they would not be as suitable for the purposes of the present invention as disclosed hereafter.